A relatively wide variety of materials can be used in an inkjet method, and the method dose not require manufacturing a photomask or a die in advance. A conventional inkjet recording method in which an ink drop is made to flow at an appropriate time has been known as a drop-on-demand method or the like, such as a piezoelectric conversion method, a bubble jet (registered trademark) method, or a thermal conversion methods of a thermal system or the like (for example, JP-B-61-5991). Various methods have been used in efforts to manufacture a three-dimensional structure using an inkjet or the like.
However, in the case of manufacturing a three-dimensional structure using an inkjet, there are some problems to be solved. One is that, because it is usually in a liquid state that is ejected by the inkjet, an ejected liquid itself can not be stacked up in three-dimensions. For this reason, in order to stack the ejected liquid, another means is required to solidify it.
A method is proposed of which a photo-setting resin is used as an ejection liquid, and the liquid drop is solidified after landing on a substrate. Further, a method of manufacturing a three-dimensional structure by ejecting a binder such as water onto a substrate made of powder such as plaster, to solidify the material of the substrate, has also been practiced. However, there is a limit to material to be used and the like in either of these two methods.
Moreover, a conventional inkjet recording method has the following fundamental problems to be solved to manufacture an ultra-fine three-dimensional structure.
<Difficulties in Ejection of an Ultra-Fine Droplet>
Currently, in an inkjet system (piezo system or thermal system) that is practically and popularly used, a minute amount of liquid such as smaller than 1 pl can hardly be ejected. This is because the pressure required for ejection increases as the diameter of the nozzle decreases to be finer.
<Luck of Landing Accuracy>
Kinetic energy given to a droplet ejected from a nozzle decreases in proportion to the cube of the droplet radius. For this reason, a fine droplet cannot possess kinetic energy that is sufficient to withstand air resistance, and accurate landing cannot be expected, because of air convection or the like. In addition, as the droplet becomes fine, the effect of surface tension increases, which makes the vapor pressure of the droplet become high, and drastically increases the amount of evaporation (In this invention, unless otherwise particularly specified, “evaporation” includes meaning of volatilization). With this being the case, the mass of the flying fine droplet is considerably lost and even the shape of the droplet can hardly be kept in landing.
As described above, miniaturization of a droplet and increased accuracy of landing positions thereof are incompatible subjects so that both cannot be easily realized at once.
In addition, in a special method using an inkjet method, a low-melting point solder alloy (lead-tin alloy) is ejected by a piezoelectric-type inkjet, to form a three-dimensional structure having a height of approximately 60 μm. In this method, the kinetic energy of a flying droplet is increased, to some extent, by using a solder alloy having a large specific gravity as ink. However, the diameter of the droplet is approximately from several tens of μm to 100 μm (for example, Electronics Manufacturing, MicroFab Technologies, Inc., D. J. Hayes, W. R. Cox and M. E. Grove, J. 8 (1998), 209).
Moreover, a method for forming a three-dimensional structure by an inkjet using solder utilizes the solidification of solder caused by a reduction in temperature when the solder is landed outo a substrate (for example, U.S. Pat. Nos. 5,736,074 and 6,114,187). Therefore, in the above-mentioned method, the kinds of liquids to be ejected are limited by their melting points.
Furthermore, an electrostatic coating has been known as a method for forming a film by an electric field, and this method aims to obtain a uniform strong coating film, but the method cannot achieve formation of a three-dimensional structure.
As described above, it is difficult to manufacture an ultra-fine three-dimensional structure; for example, a pillar-shaped fine structure, by a conventional inkjet method.
A fine three-dimensional structure; above all, a structure having a high aspect ratio (length ratio in the direction of height relative to the short diameter in cross section or the bottom surface of a structure (height/diameter in cross section)), is applicable to many uses, including nano technology, for example, through holes and bumps in a surface-mounted substrate and the like. For example, a method for manufacturing a laminated integrated circuit unit, by utilizing a pillar-shaped three-dimensional structure having a cross-sectional diameter of 25 μm as a bump, is disclosed in U.S. Pat. No. 6,114,187. However, if a three-dimensional structure can be formed with high accuracy and can be further reduced in size, an integrated circuit having smaller size and higher integration density can also be manufactured.
Meanwhile, an electrohydrodynamic inkjet having a fine nozzle has been developed to realize the ejection of a minute amount of liquid (for example, JP-T-2002-538461 and JP-A-2001-38911). However, both of these technologies aim to take out a minute amount of liquid in a liquid state or a sprayed state, and hence they cannot be applied to the formation of a three-dimensional structure. Moreover, the diameter of a nozzle disclosed in the patent documents is several tens of μm and cannot be satisfactory in terms of forming a finer three-dimensional structure.
As described above, inkjet and methods for forming a three-dimensional structure by utilizing the same have been proposed, but they cannot be said to be sufficient in terms of the accuracy with which a liquid drop is landed on a substrate, the fine fabrication of a three-dimensional structure, and materials to be used. Therefore, a method for solving these problems has been required.
Other and further features and advantages of the invention will appear more fully from the following description, taken in connection with the accompanying drawings.